Elsevier

Scientia Horticulturae

Volume 162, 23 October 2013, Pages 125-134
Scientia Horticulturae

A model approach revealed the relationship between banana pulp acidity and composition during growth and post harvest ripening

https://doi.org/10.1016/j.scienta.2013.08.011Get rights and content

Highlights

  • Models to predict banana pulp acidity based on acid/base reactions are proposed.

  • The models allowed predictions of pulp acidity during fruit growth and post harvest ripening.

  • Organic acids and potassium are the main contributors to banana pulp acidity.

Abstract

Titratable acidity and pH are important chemical traits for the organoleptic quality of banana since they are related to the perception of sourness and sweetness. Banana fruit has the particularity of having separate growth and ripening stages, during which pulp acidity changes. A modeling approach was used to understand the mechanisms involved in changes in acidity during pulp growth and post harvest ripening. Changes in pH and titratable acidity were modeled by solving a set of equations representing acid/base reactions. The models were built using data from growth and post harvest ripening of three dessert banana cultivars with contrasting acidity. For each model, calculated values were compared to observed values. These models allowed the prediction of pH (R2 = 0.34; RMSE = 0.75, biais = 0.05) and of titratable acidity (R2 = 0.81, RMSE = 2.05, biais = −1.44) during fruit growth and post harvest ripening. The sensitivity analyses showed that among acids, malic, citric and oxalic acids are the main contributors to banana pulp acidity, and that among soluble minerals, potassium also plays an important role. Studying the factors that affect the accumulation of organic acids (citric, malic, and oxalic acids) and potassium in banana pulp could be a relevant area of research with the objective of modifying banana fruit acidity.

Introduction

Fruit acidity is a topic of primary importance in improving fruit quality since it influences the perception of both sourness and sweetness (Bugaud et al., 2011, Esti et al., 2002). These two attributes are major drivers of consumer preferences for fruit (Lyon et al., 1993), and are thus important traits to consider in breeding programs. Understanding the elaboration of fruit acidity is also important because acidity controls numerous enzyme activities (Madshus, 1988).

Fruit acidity is commonly measured using two chemical parameters: titratable acidity (TA) i.e. the amount of weakly bound hydrogen ions that can be released from the acids, and pH, the activity of free hydrogen ions. Fruit acidity is due to the acidity of the vacuole which represents about 90% of the volume of most mature fruit cells (Etxeberria et al., 2012). The acidity of the vacuole is the result of its ionic composition, mainly organic acids and mineral cations that determine the vacuolar pH and TA (Etienne et al., 2013). Banana pulp contains three major organic acids, malic acid, citric acid, and oxalic acid, whose concentrations undergo marked changes during growth and ripening (John and Marchal, 1995, Jullien et al., 2008) and phosphoric acid (Bugaud et al., 2013). Banana pulp contains soluble minerals, mainly potassium (K), and to a lesser extent magnesium (Mg), calcium (Ca), and chloride (Cl) (John and Marchal, 1995). During post harvest ripening, mineral content can still change due to migration between the peel and the pulp (Izonfuo and Omuaru, 1988).

There are considerable differences in pH and TA among dessert banana cultivars and among post-harvest ripening stages (Bugaud et al., 2013, Chacón et al., 1987), and the origins of these differences remain unclear. Quantifying the relations between pulp acidity and pulp ionic composition using a modeling approach, would advance our understanding of the determinants of banana acidity. Models of pH and TA predictions have been developed for peach (Lobit et al., 2002) and proved to be powerful tools to understand the mechanisms underlying changes in acidity during peach development. The objective of the present work was to apply and validate these models on banana fruit, in which other ionic species than those found in peach need to be taken into account, and to throw light on the determinants of the changes in pH and TA that occur during the life of the banana pulp, i.e. from growth on the plant through post harvest ripening.

Section snippets

pH model

The model used for pH prediction was adapted from Lobit et al. (2002). Banana pulp can be considered as a concentrated aqueous solution of weak acids, mainly malic, citric, oxalic and phosphoric acids, and mineral cations, mainly potassium, magnesium, calcium and chloride. Other acids can be found in banana pulp but were not taken into account in the present study. Weak acids are partly in free form and partly dissociated to form salts with monovalent cations. Proton exchange reactions occur

Changes in pulp acidity

Fruit age and the cultivar had a significant effect on pH and TA during fruit growth (Table 1). Throughout fruit growth, PL had the most acidic fruits (TA = 3.5 mEq 100 g FW−1 ± 0.22; pH = 5.5 ± 0.13), IDN 110 fruits were intermediate (TA = 2.8 mEq 100 g FW−1 ± 0.31; pH = 5.7 ± 0.17), and JB had the least acidic fruits (TA = 2.3 mEq 100 g FW−1 ± 0.27; pH = 5.9 ± 0.28) (Fig. 1A and B). In all three cultivars, TA decreased slightly during the early stages of fruit growth and then increased slightly. pH increased throughout fruit

Quality of prediction of the models

For the pH model, the lower the pH, the better the predictions, which explains why pH predictions were better during ripening than during fruit growth. This is due to the logarithm function of the pH which increases the sensitivity of the pH to input parameters with an increase in pH. Thus, imprecision in the determination of the chemical elements that are the main contributors to banana pulp acidity (organic acids and K) may be responsible for the difference between observed and predicted

Conclusions

This study, which presents a model of banana pulp acidity for the first time, showed that among acids, malic, citric and oxalic acids are the main contributors to banana pulp acidity, and that among soluble minerals, K also plays an important role. Consequently, studying the factors that affect malic acid, citric acid, oxalic acid, and K accumulation in banana pulp appears to be an appropriate area of research to ultimately modify banana fruit acidity. In future work, the pH model will be

Acknowledgment

Financial support for this study was provided by Structural European Funds.

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